The present invention is directed to an expansion dowel formed of an axially extending sleeve having an axially extending through bore, an axially extending expanding region with at least one axially extending slot and an expanding member axially displaceable in the through bore from an initial position to an end position.
Conventional expansion dowels comprise an expansion sleeve with a through bore and an expansion member axially displaceable in the through bore. The expansion sleeve has an expansion region provided with at least one axially extending slot, with the expansion region extending to the leading end of the expansion sleeve, that is the end inserted first. Usually the expansion region is divided into four expansion tangs separated from one another by axially extending slots. The through bore is cylindrically shaped in the portion of the expansion sleeve extending from the expansion region to a trailing end and serves as a load engagement part. The through bore can also be cylindrically shaped in the expansion region, however, often it tapers in a cone shaped manner towards the leading end of the expansion region. The expanding member is usually retained in a captive manner in an initial position in the through bore. The expanding member can be cylindrically shaped, cylindrical with an adjacent conically shaped part or spherically shaped for its axial extent.
The spreading of the expansion dowel occurs by the axial displacement of the expanding member in the through bore from its initial position into an end position. The expansion region is widened with the expansion tangs pressed outwardly against the wall of the receiving bore. Special setting tools are used in the expanding step in which the expanding member is driven into a predetermined position by axially directed blows. The axial blows are usually exerted directly on the expanding member, so that it is driven in the setting direction in the through bore while widening the expansion region. It is also possible that the expansion sleeve is driven over an expanding part projecting inwardly from its leading end.
In known expansion dowels, the through bore is rotationally symmetrical in the unexpanded state of the dowel. The radius of curvature of the expansion dowel and of the expanding member are matched to one another in a contact region in the initial position of the expanding member. The expanding member, retained captively in this manner in its initial position in the through bore, is completely embraced by the wall of the through bore and in its initial position in the contact region usually has an area-type contact with the wall of the through bore. The tangs of the expansion region are pressed outwardly due to the axial displacement of the expanding member into its end position. The through bore loses its rotationally symmetrical shape during the expanding step. Since the expanding member continues to have a rotationally symmetrical outer contour, it has essentially only a line-shaped contact in its end position in the contact region with the expansion tangs. The high expanding pressure must be transmitted over a very small contact region and locally there is developed high area pressures which lead to a slight adaptation of the radii of curvature of the expansion tangs and the expanding member in the contact region. Accordingly, additional energy must be supplied, and does not assist in the expanding operation. High frictional forces must be overcome in displacing the expanding member from its initial position into its end position, causing an increase in the required setting energy. Due to the high frictional forces, a coating applied upon the expanding member, for instance, in order to protect it from corrosion can be scraped off and thereby becomes useless. If rust free materials are used this can lead to cold welding in view of the high frictional forces generated during displacement of the expanding member. The expansion sleeve, however, is only expanded in an incomplete manner and the load attachment region cannot attain the required load carrying values or it may be necessary to completely abandon the dowel.